1. Field of the Invention
This invention relates to an engine speed control system for an outboard motor.
2. Description of the Related Art
In an outboard motor to be used as a propulsion unit of boats, when a trouble such as overheating occurred in its internal combustion engine, the engine must continue to run until the boat has returned to port.
Aside from the above, in recent outboard motors, fuel injection or ignition is electronically controlled through an electronic control unit (ECU), although steering, throttling and shift-changing are still mechanically controlled using a cable or link. In such a type of the outboard motors, as taught in Japanese Laid-Open Patent Application No. 2001-41079, when a trouble has occurred in the engine (in the prior art, the trouble is a failure of sensor for sensing operating conditions of the engine), the ECU retards ignition timing, thins out the number of ignitions, or discontinues ignition at a certain cylinder if the engine is multi cylinder engine, so as to lower the engine speed such that the engine is protected from being damaged.
However, in the prior art, since the engine speed is lowered by retarding ignition timing or by decreasing the number of ignitions, the engine is likely to vibrate and this makes the operator feel unpleasant.